With the development of information and communications technology (ICT) and digitization of electronic devices, image sensors having improved performance have been used in, for example, digital cameras, camcorders, portable phones, personal communication systems (PCSs), game machines, security cameras, and medical micro cameras.
An image sensor may include a photodiode configured to convert an optical image into an electric signal and transistors (e.g., a transfer transistor, a reset transistor, and a sensing transistor) configured to transmit signals sensed by the photodiode to a signal processing circuit. With an increase in the integration density of semiconductor devices, image sensors have been gradually highly integrated. Thus, each of pixels has been downscaled, thereby increasing the likelihood of image lag.